Relationship between Land Use and Erosion Intensity in Ciwidey Watershed West Java

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1 Nana S., Achmad S., Emi S./ICG 0 Relationship between Land Use and Erosion Intensity in Ciwidey Watershed West Java Nana Sulaksana, Achmad Sjafrudin and Emi Sukiyah Faculty of Geology, Padjadjaran University, Jalan Raya Bandung-Sumedang Km, 46, Bandung, Indonesia nana_s@unpad.ac.id Abstract Ciwidey watershed is located in the southern part of Bandung regency. In the upstream area, there are some tourism places. These tourism places have implications for expansion of land use change for the construction of tourism support facilities. This research aims to determine the effect of land use in Ciwidey watershed against erosion intensity. A Landform has various land use. Remote sensing data and topographic map are used for delineating land use. Based on previous publication data, average erosion intensity and erosion total can be calculated. Probabilistic approach is also used in analysis. Diversity of rocks and tectonic activity can be reflected by bifurction ratio (Rb). Most of the watersheds have Rb value less than. It shows that variation of rocks and tectonic are complex. Land use distributions are forests (6%), plantations (8%), fields (%), shrubs (%), irrigated paddy fields (0%), rainfed paddy fields (4%), settlements (9%), and empty land (less than %). Amount of total erosion is influenced by subwatershed area and land use variation. Average erosion intensity in Cimonce subwatershed is higher than other subwatersheds (.46,9 tons/ha/year). The lowest erosion intensity is in Cigadog subwatershed (, tons/ha/year). If the ratio of cultivated land increases, the erosion intensity will increase. In contrary, the correlation between the ratio of forest and erosion intensity is inversely proportional. If the ratio of forest increases, the erosion intensity will decrease. The balance of land use as cultivation and forest areas should be monitored properly. Keywords :Ciwidey, erosion, land use, morphotectonic, watershed. Introduction Environmental quality degradation can occur as a result of land use conversion that does not consider the natural carrying of the land. A watershed has limitations to manage water resources. In order to maintain the balance of water supply, land use in the watershed must also be regulated properly. If the balance is disturbed, between the input and output of water in the watershed, the negative impact of catastrophic environmental damage cannot be avoided. Ciwidey watershed is one of the subwatershed in upstream Citarum watershed, located in the southern part of Bandung regency. There are tourism places in the upstream of Ciwidey watershed, Patuha crater and Situ Patenggang. These tourism places have implications for the expansion of land use change for the construction of hotels and tourism support facilities. On the other hand, the floods hazard always occurs in the downstream of Ciwidey watershed, Soreang and surrounding area. The cause of flooding in the downstream area is the inability of the Ciwidey river accommodate runoff, especially when rainfall intensity relatively high. This is presumably because there is a narrowing and silting of river channel that happens so fast, due to the uncontrolled erosion as a result of rapid land use conversion. Research activities related to the geomorphology and erosion-floods has been done by some previous researchers, Sukiyah et al. (006), Sukiyah et al. (0), Sulaksana et al. (0), and Sulaksana et al. (0). The result of those researches showed that an association between morphometric, morphotectonic, erosion, silting of rivers, and floods of watershed. However, detailed research on land use change and its impact on the increase of erosion intensity have not been done before in this area. This research aims to determine the role of land use in the Ciwidey watershed can affect the 9

2 Nana S., Achmad S., Emi S./ICG 0 erosion intensity. Besides, this research aims to determine the existing land use, calculate the percentage of land use, calculate the erosion intensity, and determine the role of land use change on the erosion intensity. The results are expected to be useful to help the community and local government to solve development issues. Erosion which passes its limit can cause deposition of material in the lower elevation areas. This deposition can lead to silting of the river channel. At a certain time, the river flow was not able to accommodate run off so that it will lead to floods in the surrounding area (Ilyas, 987). Erosion, transport, and sedimentation are complex processes and interrelated. Erosion is an exogenous process that takes place gradually, caused by water, wind, and snow activity (Field and Engel, 004). Arnoldus (974; in Swaify et al., 98) divides erosion into two types, geological erosion and accelerated erosion. Geological erosion occurs naturally and lasts for geological time ranges from thousands to tens of millions years. This erosion type generally balances with the natural phenomenon. The accelerated erosion caused by human activity, generally changes the natural condition drastically such as land use change that are not regulated properly due to the development processes. Floods are an incident where the normally dry land becomes flooded land due to heavy rainfall. Topographic factor such as lower elevation to concave area and low soil infiltration ability which causes the inability to absorb water are other factors causing floods. In addition, floods can also be defined as an overflow of river water due to inability river channel to accommodate runoff. Erosion caused by water activities can be classified into several types (Thornburry, 969): splash erosion sheet erosion rill erosion gully erosion The amount of eroded material can be measured in the field by placing the measuring tools in several locations that have been assigned before. The prediction of the material eroded amount can also use erosion equation. USLE (Universal Soil Lost Equation) is a calculation formula for predicting annual sheet erosion (Wischmeier and Smith, 96; in El-Swaify et al., 98), which is formulated as follows: A = RKLSCP.() with A = the average annual soil loss (tons/acre), R = run off erosivity index, K = soil erodibility factor, LS = topographic factor (L = slope length factor, S = slope steepness factor), C = croppingmanagement factor, P = erosion control practice factor. Land management factors play an important role in increasing the erosion intensity. 96 Methodology Research objects are landform, river, land use, and erosion. These objects can be identified through various media such as remote sensing data, topographic map, and field observation. Research area located in Ciwidey watershed (Figure ) is part of upstream Citarum watershed. The research area administratively is located in Bandung district, West Java. Figure. Research area location Some of the variables that are not measured in the field is determined through satellite imagery and topographic map. Methods of the interpretation and acquisition of information from remote sensing imagery are:

3 Nana S., Achmad S., Emi S./ICG 0 Visual interpretation based on tone, pattern, shape, texture, etc. Remote sensing data integration with other data types. Erosion intensity can be obtained from USLE formula (). Basic data used in this research derived from scientific publications of some researchers equipped with field observation data and laboratory analysis. Data analysis method used in this research is using probabilistic approach. This method is used in order that the research results have particular significance level, for example = to = 0,0. Before analyzed, the data need to be determined the level of distribution normality using the Liliefors test method. After the data distribution known to be a normal data that reflects its population, regression-correlation analysis conducted to determine the level of relationship between the related parameters. Research framework is shown in Figure. Tertiary epiclastic sedimentary rocks and alluvial deposit (Alzwar dkk., 99; Koesmono dkk., 996; Silitonga, 00). Ciwidey watershed shape is fairly complex, in the upstream and middle of watershed is relatively wide whereas in the downstream of watershed is relatively narrow. There is the significant tectonic control, resulting in an extreme watershed shape change in the downstream. Morphometry Ciwidey watershed can be divided into several subwatershed by tributaries that flow toward Ciwidey river. Based on data analysis, the watershed is divided into subwatershed as listed in Table and shown in Figure. The rivers within each subwatershed are given the order to determine the morphometric characteristic. Based on Table, river order for each subwatershed has different distribution. Cisondari and Cicangkorah subwatershed have river order more than other subwatershed, ranges from to whereas the least river order and river segments are Cimonce and Garung watershed, only have river order ranges from to. Variation of river order can reflect the subwatershed area. Figure. Research framework Result and Discussion Geological feature of Ciwidey watershed is very complex, composed by various volcanic rocks, epiclastic sedimentary rocks, and alluvial deposits with active tectonic feature. The upstream and middle of watershed are composed by Quaternary volcanic rocks whereas the downstream of watershed is composed by Table. Distribution of river order and Bifurcation ratio (Rb) in Ciwidey watershed Sub watershed Order Rb ,09,4,8,0,,79,0,40,00,70,0 Cimonce,00 Garung-,0 Garung- Cisondari Ciwidey Cigadog Cicangkorah ,00 6,67 0,60 97

4 Nana S., Achmad S., Emi S./ICG 0 Siluman 4 Cinangsi 7 Kutawaringin 0 7 Cidano 7,,7,,4,,00 river upstream is forest area whereas shrubs and farm area on the gentle slope. Distribution of land use is calculated for subwatershed located in Ciwidey watershed (Table ). Variations of land use for each subwatershed begin to decrease towards downstream area. Ciwidey upstream area is a tourism place so that it appears the land use as cultivation area is growing rapidly (Figure 4). This phenomenon is also reflected by distribution of settlements area quite high. Table. Distribution of land use in Ciwidey watershed Sub watershed Figure. Subwatersheds distribution in Ciwidey watershed Besides, diversity of rocks and tectonic can also be reflected by bifurcation ratio (Rb). If variations of rocks are quite diverse and tectonic activity controls subwatershed, the Rb value will be less than or more than. Most of subwatersheds located in Ciwidey watershed have a range of Rb value less than (Table ). It indicates that the variations of rocks and tectonic activity are quite diverse and complex. Land Use Distribution of land use in the Ciwidey watershed are quite varied, forests (6%), plantations (8%), farms (%), shrubs (%), irrigated paddy fields (0%), rainfed paddy fields (4%), settlements (9%), and empty land (less than %). Data obtained from topographic map, SRTM imagery, and field observation data compilation. Based on the result of calculation, the distribution of land use proportion is quite good and deserves to be maintained. Ciwidey 98 Land use (ha) IndstrShrubs Forest Stlmnt Farms Irrgtd Rnfd Empty Moors pdy-fld pdy-fld land Cimonce 9, 44,6, 8,7 Garung- 6, 4,,7 Garung- 4, 44,,9 6, Cisondari 07, 7,9.749,0 64,,4 8,6 98,7 6, Ciwidey 98,9 947,.8 80,8 967,6 6, Cigadog, 46,7 744,9 89, 86,9 769,0 7, Cicangkorah 8,6 8,0 0,.9 60,8 0,9 88,4 Siluman, 7,9 0,6,,4,6 Cinangsi, 7,8 9,6 40,9 8, 89,0 Kutawaringin 9,9 70,9 76,9 7, 7,7, Cidano 60, 8,0,0 9,8 + Figure 4. (a) Settlement area in the upstream of Ciwidey river, (b) River channel in the downstream of Ciwidey river Erosion Total erosion calculation result for each subwatershed is shown in Table. Total erosion will depend on the area of each subwatershed. The larger of a subwatershed in an area, the greater of total erosion material is produced (Figure ). Graphic on Figure is obtained after the data showed that the logarithm data

5 Nana S., Achmad S., Emi S./ICG 0 distribution of subwatershed area and logarithm data of total erosion is normal. This is indicated by the critical value of calculation result (L) is always smaller than Ltabel ( = 0,) with data. In addition to the total erosion, average erosion intensity for each hectare in each year is also known. Cimonce subwatershed has the highest average erosion intensity, reached.46,9 tons/ha/year (Figure 6). High intensity of erosion in this area is caused by land use in Cimonce subwatershed which is entirely cultivated area. Meanwhile, the lowest average erosion intensity in Cigadog subwatershed is, tons/ha/year. this phenomenon is associated with more varied land use. In addition, the ratio of forest area is still adequate. Table. Ratio between variation of land use and total erosion in Ciwidey watershed Sub watershed Area (ha) Av.ersn Rat. Rat. Rat. Rat. Tot.erosion Av.ersn (tons/ha/ frst cltvtn frms moors (tons/yr) log. yr) Cimonce 60,,0 0, 0, 46, 90.7,,4 Garung- 6,9,0 0,7 0, 647, 0.68,, Garung-,,0 0,4 0,6 89, 7.8,0, Cisondari 6. 0, 0, 0, 0, 749, ,,9 Ciwidey 4.66,0 0,4 0,4 0, 0, 64,.99.48,6,8 Cigadog.6,0 0, 0, 0,,.9.640,,7 Cicangkorah.4 0,9 0,4 0, 6, ,8, Siluman 79,4 0,9 0,6 0, 868,.74,8, Cinangsi 4, 0,8 0,4 0, 06,9 4.74,, Kutawaringin 9,8 0,9 0, 0, 987,.07.68,, Cidano,0 0,4 0, 746, 44,.89,7,9 Figure. Correlation between subwatershed area logarithm and erosion logarithm shows equation: y = 0,74x +,779 with r=0,96 Figure 6. Average erosion distribution in each subwatersheds for each unit area in each year Role of Land Use to Erosion Intensity Distribution of land use can affect the erosion intensity. It can be proved by the result of analysis with probabilistic approach. After all data associated with the erosion have normal distribution, the regression test is conducted to determine the level of the relationship between these variables. Ratio between variation of land use and subwatershed area is calculated and compared with the average erosion intensity for each subwatershed (Table ). Based on analysis result, the correlation between the moor area ratio and average erosion logarithm shows the equation: y = 0,848x +,94 with r = 0,66 (Figure 7A) whereas the correlation between the farm area ratio and average erosion logarithm shows equation: y = 0,794x +,80 with r = 0,7 (Figure 7B). If the entire cultivation area ratio calculated and linked to the average erosion logarithm, the equation will be: y = 0,79x +,497 with r = 0,7 (Figure 8). Those three graphics shows the relationship is directly proportional to the correlation coefficient ranged from strong to very strong. The phenomenon shows that if the ratio of cultivated land use (settlement, farm, and moor) increases, it will increase the average erosion intensity in the area. 99

6 Nana S., Achmad S., Emi S./ICG 0 Figure 7. (A) Correlation between moor ratio and average erosion logarithm shows equation: y = 0,848x +,94 with r= 0,66, (B) Farm ratio and average erosion logarithm shows equation: y = 0,794x +,80 with r=0,7 Different from cultivation area, the correlation between the ratio of forest area and average erosion intensity shows the equation: y = -0,6064x +,06 with r=0,7 (Figure 9). This phenomenon shows that if the distribution of forest area ratio increases, the average erosion intensity is predicted to decrease. It is based on the understanding that there are plants in forest area have strong roots and is able to withstand the rate of surface erosion. Forest area generally also has lush leaves so it can slow down the rainwater that fall to soil surface. The balance of land use between cultivation area and forest area should be properly monitored and carried out continuously. Along with the increase in population, the demand for cultivation area would increase as well. Therefore, engineering technology in the field of infrastructure development can help to reduce negative risk of development impacts. Conclusion Ciwidey watershed has potential to be developed, such as the tourism places in the upstream of Ciwidey watershed. This condition has an impact on the increase of land use for cultivation area. Monitoring of land use balance between cultivation area and forest area is an urgent and important activity. The imbalance between land uses can disrupt the natural balance of watershed. One of the consequences of this imbalance is the average erosion intensity that exceeds the limit. Ratio of the increase in cultivation area will be directly proportional to the increase in erosion intensity. Otherwise, a relatively high ratio of forest area would reduce the average erosion intensity in the area. Acknowledgments Figure 8. Correlation between cultivation area ratio and average erosion logarithm shows equation: y = 0,79x +,497 with r=0,7 We thank Faculty of Geological Engineering, Padjadjaran University, which have helped the research activity so this scientific article can be made. In addition, we would like to thank all those who have helped in this research activity. Hopefully, this scientific article can be useful both for the science development and society. References Alzwar, M., Akbar, N. & Bachri, S. 99. Geological Map of Garut and Pameungpeuk Quadrangle, Java, Scale : Geological Research and Development Centre, Bandung. Figure 9. Correlation between forest ratio and average erosion logarithm shows equation: y = -0,6064x +,06 with r=0,7 00 El-Swaify, S. A., Dangler, E. W. & Armstrong, C. L. 98. Soil Erosion by Water in the Tropics. Department of Agronomy and Soil Science, University of Hawaii.

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